Motorized joint positioner

ABSTRACT

A motorized joint positioner includes a first holder configured for a first portion of a patient&#39;s anatomy to rest thereon, and a second holder configured for a second portion of the patient&#39;s anatomy to rest thereon. The joint positioner further includes a first robotic arm coupled to the first holder, and a second robotic arm coupled to the second holder. At least one of the first and second robotic arms comprises at least two joints providing movement of the corresponding first or second holder in at least two degrees of freedom. At least one of the first and second robotic arms includes an actuator controllable to position the corresponding first or second holder.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority to U.S. ApplicationNo. 61/747,792, filed Dec. 31, 2012, which is incorporated by referenceherein in its entirety.

BACKGROUND

The present disclosure relates generally to the field of devices used toposition a joint in connection with a medical procedures. Devices thathold a portion of a patient's anatomy (e.g., arm holders, leg holders,etc.) are useful in the medical industry to position and stabilize apatient's joint.

Some drawbacks of conventional joint positioners include the time andeffort required to adjust the position of the patient's joint. Forexample, some joint positioners require a user to perform a mechanicaladjustment to adjust the joint position. Adjusting such a positionerincreases the time required to complete the medical procedure, and usersmay find it cumbersome or difficult to achieve a precise adjustment.

SUMMARY

The present application is directed to a motorized joint positioner thatallows a user to quickly and accurately position a patient's joint. Auser may wish to position a patient's joint, for example, in a doctor'soffice or during a surgical procedure. The user can easily andeffectively maintain or adjust the position of the patient's joint usingthe motorized joint positioner described herein, which may increaseefficiency in the doctor's office or operating room. In one embodiment,the positioner may be used to assist a surgeon during soft tissuebalancing. In other embodiments, the motorized joint positioner includesfeatures that are advantageous when the motorized joint positioner isbeing used in connection with a computer-assisted surgery (“CAS”)system. These features include a local tracking system to track aportion of the patient's anatomy and a two-dimensional ultrasound arrayto enable registration of a portion of the patient's anatomy.

According to one exemplary embodiment, a motorized joint positionerincludes a first robotic arm coupled to a first holder and a secondrobotic arm coupled to a second holder. At least one of the first andsecond robotic arms includes an actuator controllable to position thecorresponding first or second holder.

According to another exemplary embodiment, a motorized joint positionersystem includes a motorized joint positioner having first and secondrobotic arms, wherein at least one of the first and second robotic armsincludes a series elastic actuator. The system further includes aprocessing circuit configured to position the motorized joint positionerby controlling the series elastic actuator. The motorized jointpositioner is backdrivable such that it can be manually repositioned bya user.

Alternative exemplary embodiments relate to other features andcombinations of features as may be generally recited in the claims.

BRIEF DESCRIPTION OF THE FIGURES

The disclosure will become more fully understood from the followingdetailed description, taken in conjunction with the accompanyingfigures, wherein like reference numerals refer to like elements, inwhich:

FIG. 1 is a side view of a motorized joint positioner, according to anexemplary embodiment.

FIG. 2 is a top view of the motorized joint positioner of FIG. 1.

FIG. 3 is a motorized joint positioner holding a patient's knee in anextended position, according to an exemplary embodiment.

FIG. 4 is an exemplary embodiment of a computer-assisted surgery systemthat includes a motorized joint positioner.

FIGS. 5A and 5B illustrate a motorized joint positioner being utilizedfor soft tissue balancing, according to an exemplary embodiment.

DETAILED DESCRIPTION

Exemplary Motorized Joint Positioner

Before turning to the figures, which illustrate the exemplaryembodiments in detail, it should be understood that the application isnot limited to the details or methodology set forth in the descriptionor illustrated in the figures. It should also be understood that theterminology is for the purpose of description only and should not beregarded as limiting.

The motorized joint positioner described herein can be used in anycontext to position a joint. For example, surgeons may use the motorizedjoint positioner prior to or during a surgical procedure. Other medicalprofessionals may use the motorized joint positioner during examination,testing, or imaging of the joint. The motorized joint positioner canalso be used to position a variety of different joints, such as a knee,hip, ankle, elbow, shoulder or wrist.

Referring to FIGS. 1 and 2, a motorized joint positioner 100 accordingto an exemplary embodiment includes a first holder 2 and a second holder4. The first holder 2 is coupled to a first robotic arm 6 and the secondholder 4 is coupled to a second robotic arm 8. The robotic arms 6, 8 aredriven by actuators, such as series elastic actuators (SEAs). In thefigures, the motorized joint positioner 100 is illustrated duringpositioning of a knee.

The first and second holders 2, 4 can be any suitable structure forholding a portion of a patient. In the embodiment shown in FIG. 1, theholders 2, 4 are cuffs. The patient's upper leg (e.g., thigh) rests inholder 2, and the patient's lower leg (e.g., ankle) rests in holder 4.The holders 2, 4 may further include one or more straps 10 for securingthe patient to the holders 2, 4.

Positioning

The robotic arms 6, 8 can each include one or more SEAs. The SEAs may beany commercially available SEA and may be rotational or linearactuators. The SEAs are configured to enable force-control andhigh-precision position control of the robotic arms. Multiple SEAs canbe linked, as shown in FIG. 1, to provide position control in numerousdegrees of freedom. In one embodiment, the first robotic arm 6 is a twodegree-of-freedom (DOF) robotic arm that includes a first joint 11 and asecond joint 12 (FIG. 2), and the second robotic arm 8 is a six DOFrobotic arm that includes a first joint 14, a second joint 16, a thirdjoint 18, a fourth joint 20, a fifth joint 22, and a sixth joint 24(FIG. 1). Each joint can be controlled by a corresponding SEA. However,as many joints (and corresponding SEAs to control the joints) as desiredmay be linked to form robotic arms with the desired degrees of freedom.Rotational or linear SEAs may be chosen to obtain a compact design ofthe first and second robotic arms 6, 8.

Referring to FIG. 4, the joint positioner 100 may be used in connectionwith a CAS system 200. The CAS system 200 may include, among otherthings, the motorized joint positioner 100, a processing circuit(represented in the figures as a computer 26), a secondary trackingsystem 28, and a haptic device 30. The haptic device 30 is aninteractive robotic device used by a surgeon during a surgicalprocedure, such as the robotic device described in U.S. Pat. No.8,010,180, titled “Haptic Guidance System and Method,” which is herebyincorporated by reference herein in its entirety.

The joint positioner 100 can be controlled (e.g., by computer 26 ormanually by a user) to position the patient's joint. For example, thepatient's knee may be brought from the flexed position shown in FIGS. 1and 2 to the fully extended position shown in FIG. 3. The computer 26may control the motorized joint positioner 100 to bring the holders 2, 4(and thus the portion of the patient held by the holders 2, 4) to adesired position based on a preoperative surgical plan. During asurgical procedure, the computer 26 can control the motorized jointpositioner 100 to bring the holders 2, 4 to positions corresponding todifferent stages of a surgical plan. For example, if a certain stage ofa knee replacement surgery requires the femur and tibia to be pulledaway from each other, the computer 26 can be programmed to control themotorized joint positioner 100 to accomplish this positioning.

The force control capabilities of the SEAs enable the motorized jointpositioner 100 to fully compensate for the weight of the patient'sextremity or other body part held by the positioner 100. In oneembodiment, the motorized joint positioner 100 applies forces to thefirst and second holders 2, 4 to counteract the weight of a portion of apatient's anatomy held by the motorized joint positioner (e.g., thepatient's leg). This gravity compensation feature causes the portion ofthe patient to feel weightless as a user is manually repositioning thejoint positioner 100. Consequently, the user is able to manuallyreposition the joint positioner 100 without having to exert additionaleffort to lift or move the weight of the portion of the patient'sanatomy. The backdrivability of the SEAs further contribute to the easewith which a user can manually adjust the motorized leg positioner 100(i.e., manually adjust the position of the first and second holders 2,4).

In one embodiment, the motorized joint positioner 100 has two modes. Inthe first mode, the positioner 100 may be utilized to hold the joint ina fixed position. This first mode is useful, for example, while asurgeon is using haptic device 30 to sculpt or otherwise modify thepatient's joint. In one embodiment, the CAS system 200 is programmed tohold the positioner 100 in a fixed position while the haptic device 30is in a cutting mode (i.e., when the surgical tool coupled to the hapticdevice is being operated). The surgeon can set the desired position ofthe joint positioner 100 by manually positioning the joint positioner100 and then alerting the CAS system 200 to fix the position of thejoint positioner 100. For example, once the joint positioner 100 is in adesired position, the surgeon can use input device 58 to set the jointpositioner 100 in the first mode, thus fixing the joint positioner 100.The motorized joint positioner 100 may have a second mode in which auser can manually adjust the position of the motorized joint positioner100. This second mode is useful during steps of a medical procedure inwhich it is advantageous for a surgeon to reposition the patient'sjoint, such as during surgical planning. The SEA actuators within therobotic arms 6, 8 provide a backdrivable system, allowing the user tomanually manipulate the positions of the first and second holders 2, 4.The CAS system 200 determines how much force is required to compensatefor the weight of the patient's leg and can sense incremental changes inforce as a user manipulates the position of the joint positioner 100.

The processing circuit of the CAS system 200 is utilized to implementthe various functions (e.g., calculations, control mechanisms,processes) described herein, such as computerized control of themotorized joint positioner 100. The processing circuit includes aprocessor and memory. The processor can be implemented as a generalpurpose processor, an application specific integrated circuit (ASIC),one or more field programmable gate arrays (FPGAs), a group ofprocessing components, or other suitable electronic processingcomponents. The memory (e.g., memory, memory unit, storage device, etc.)is one or more devices (e.g., RAM, ROM, Flash memory, hard disk storage,etc.) for storing data and/or computer code for completing orfacilitating the various processes and functions described in thepresent application. The memory may be or include volatile memory ornon-volatile memory. The memory may include database components, objectcode components, script components, or any other type of informationstructure for supporting the various activities and informationstructures described in the present application. According to anexemplary embodiment, the memory device is communicably connected to theprocessor and includes computer code for executing one or more processesdescribed herein.

The joint positioner 100 may communicate with the computer 26 via acommunication interface. The communication interface can be or includewired or wireless interfaces (e.g., jacks, antennas, transmitters,receivers, transceivers, wire terminals, etc.) for conducting datacommunications with external sources via a direct connection or anetwork connection (e.g., an Internet connection, a LAN, WAN, or WLANconnection, etc.). For example, the communication interface can includean Ethernet card and port for sending and receiving data via anEthernet-based communications link or network. In another example, thecommunication interface can include a WiFi transceiver for communicationvia a wireless communications network.

Soft Tissue Balancing

Referring to FIG. 5, the motorized joint positioner 100 may enablecomputerized soft tissue balancing during joint repair surgeries. Forexample, an important part of some knee replacement surgeries isachieving an acceptable balance of the ligaments surrounding the knee.Conventionally, to achieve the desired soft tissue balance during asurgical procedure, a surgeon manually manipulates the patient's knee todetermine the tension of soft tissue during various movements. The jointpositioner 100 described herein can assist the surgeon during balancingby moving the patient's joint through a range of motion and providingmeasurements representative of the tension in the patient's joint. Anexample of a measurement representative of the tension in a patient'sjoint is the distance of gap 32 between two bones of the joint (e.g.,between the patient's femur and tibia). The joint positioner 100 is ableto provide known forces or torques to the patient's joint to obtainmeasurements related to the joint's tension.

In one embodiment, the joint positioner 100 moves the patient's jointprior to surgery to obtain a preoperative set of data representative ofthe patient's soft tissue balance. The preoperative set of data mayinclude, for example, the distance of gap 32 in the patient's knee whenthe joint is in a neutral position (FIG. 5A) and when a known amount oftorque is applied to the patient's knee (FIG. 5B). Preoperative data mayfurther include forces acting on the joint, such as forces resistingmovement, while the patient's joint is being positioned or guidedthrough a range of motion. The joint positioner 100 can then be usedduring a surgical procedure to move the patient's joint with a trialimplant in place, deriving another set of data. The surgeon can comparethe data obtained with the trial implant to the preoperative data anduse the information, for example, to choose an alternative shape or sizeof implant or to make other modifications to the knee. Once the surgeonbelieves the surgical operation is complete, the joint positioner 100can be used to obtain a final set of data representative of thepatient's postoperative soft tissue balance. In this manner, the jointpositioner 100 helps surgeons obtain a desired soft tissue balance,which may be similar to or different from the balance of the patient'spreoperative joint.

Local Tracking System

In one embodiment, the joint positioner 100 includes a local trackingsystem 34 to track a portion of a patient's anatomy (e.g., the portionheld by the joint positioner 100) relative to the joint positioner 100.The tracking system 34 can be optical or mechanical. In FIG. 1, thetracking system 34 is an optical tracking system that includes at leastone detection device 36 and trackable markers 38, 40. The trackingsystem 34 may further include a second detection device 37. Thedetection devices 36, 37 are fixed to the first and second holders 2, 4,respectively. The trackable markers 38, 40 are fixed to the portion ofthe patient's anatomy (e.g., the patient's bones) held by the jointpositioner 100 and are detectable by the detection devices 36, 37. Inone embodiment, the detection devices 36, 37 include a visiblelight-based detector, such as a MicronTracker (Claron Technology Inc.,Toronto, Canada), that detects a pattern (e.g., a checkerboard pattern)on the trackable markers 38, 40. As is known, the trackable markers 38,40 may be active (e.g., light emitting diodes or LEDs) or passive (e.g.,reflective spheres, a checkerboard pattern, etc.) and have a uniquegeometry (e.g., a unique geometric arrangement of the markers) or, inthe case of active, wired markers, a unique firing pattern. Thetrackable markers 38, 40 are affixed to the tracked objects (e.g., thepatient's bones), in a secure and stable manner. In one embodiment, thetrackable markers 38, 40 are fixed to the patient's bones with bone pins42, 44. In operation, the detection device 36 detects positions of thetrackable markers 38, 40, and the pose of the tracked object (e.g., thepatient's bones) relative to the detection device(s) 36, 37 can becalculated based on the trackable elements' positions, unique geometry,and known geometric relationship to the tracked objects. In this manner,the pose of the tracked objects (e.g., the patient's bones) can becalculated relative to the joint positioner 100.

In another embodiment, the detection device 24 is a three-dimensionaltracking sensor, such as the 3D tracking sensor developed by LeapMotion, Inc. (San Francisco, Calif.). The three-dimensional trackingsensor is able to track the pose of the trackable markers 26, 28, asdescribed in U.S. application Ser. No. 13/714,066, titled “Registrationand Navigation Using a Three-Dimensional Tracking Sensor,” filed Dec.13, 2012, which is hereby incorporated by reference herein in itsentirety.

Inclusion of a local tracking system 34, as shown in FIG. 1, may provideadvantages over use of a non-local (i.e., global) tracking system totrack the portion of the patient's anatomy. Some types of globaltracking systems utilize a detection device fixed relative to anoperating room. The operating room may contain various tracked objects,such as the patient's bones and surgical tools. If the global trackingsystem is an optical tracking system, a line of sight from the trackableelements and the detection device may be required. If objects or peopleblock the path from the trackable elements to the detection device, aninterference in the tracking process may result. Use of a local trackingsystem 34, as shown in FIG. 1, minimizes line of sight issues by placingthe detection device(s) 36 in close proximity to the trackable elements26, 28. In this manner, the local tracking system 34 can continuouslytrack the position of the patient's bones, which are coupled to thetrackable elements 38, 40 via bone pins 42, 44.

In one embodiment, shown in FIG. 4, the CAS system 200 includes both alocal tracking system 34 and a secondary, global tracking system 28. Thesecondary tracking system 28 can be used to track additional objects inthe CAS system 200, and may include a secondary detection device 48 andadditional trackable markers 50, 52. In one embodiment, trackable marker50 is located on the base of haptic device 30, and trackable marker 52is located on or near a surgical tool for use during a surgicalprocedure.

The local tracking system 34 may be in communication with the globaltracking system 28 such that the position of all tracked objects in CASsystem 200 can be calculated with respect to a single coordinate frameof reference. In one embodiment, an additional trackable marker isplaced on a stationary portion of the joint positioner 100. Thisadditional trackable marker is tracked by the secondary tracking system28. The CAS system 200 can use the pose of the additional trackablemarker to correlate the coordinate systems of the local tracking system34 and the secondary tracking system 28. In another embodiment, amechanical tracking system is coupled to the motorized leg positioner100 (e.g., to the detection device 36 or to another portion of themotorized leg positioner 100). The mechanical tracking system is used totrack the motorized leg positioner 100. The CAS system 200 can useinformation from the mechanical tracking system to correlate thecoordinate systems of the local tracking system 34 and the secondarytracking system 28.

Registration

In one embodiment, the joint positioner 100 includes features useful forregistration of the patient's anatomy (e.g., the bones held by jointpositioner 100) to a three-dimensional representation of the portion ofthe patient's anatomy. The portion of the patient's anatomy isregistered to allow the local tracking system 34 (or the global trackingsystem 28) to accurately monitor the position of the portion of thepatient's anatomy during a medical procedure. In one embodiment, thejoint positioner 100 includes an XY array of ultrasound transducers 54,as described in Ser. No. 13/710,955, titled “Registration Using PhasedArray Ultrasound,” filed Dec. 11, 2012, which is hereby incorporated byreference herein in its entirety. The array 54 may be located on theinterior of one or both holders 2, 4, such that the array 54 is able toscan the patient's bone structure or soft tissue. The array iscommunicably coupled to the processing circuit (i.e., computer 26) forcontrolling the ultrasound transducers and for registering the portionof the patient's anatomy to a three-dimensional representation of theportion of the patient's anatomy. The three-dimensional representationmay be obtained by any known imaging techniques (e.g, CT or MRI).Alternatively, the three-dimensional representation may be obtainedusing an imageless system. Imageless systems include technologies thatare known in the art, such as systems utilizing statistically shapedmodels and methods of bone morphing.

In one method of registration using the motorized joint positioner 100,the location of the joint positioner 100 is known. The location of thejoint positioner 100 is known either by fixing a portion of the jointpositioner 100 (e.g., to a table 56) or by tracking the joint positioner100 with a tracking system such as secondary tracking system 28. If thearray 54 is fixed to the joint positioner 100, the location of the array54 can also be determined. The array 54 is controlled to create anacoustic wave directed towards a portion of the patient's anatomysuitable for registration (e.g., a portion of bone or soft tissue havingfeatures that can be aligned with the three-dimensional representationof the bone). Because the location of the array 54 is known, thelocation of bone scanned by the array 54 can be calculated. Thisinformation can be used to register the portion of the patient's anatomyto the three-dimensional representation.

Including an array of ultrasound transducers in the joint positioner 100advantageously allows for continuous registration of a portion of apatient's anatomy during a surgical procedure. In contrast, certainother methods of registration are typically performed prior to asurgical procedure or intermittently during a surgical procedure. Theseother methods may require the surgeon to perform steps such as using aprobe to physically contact the patient's bone. Furthermore,interruptions in tracking of the patient can cause errors inregistration, requiring the surgeon to stop the procedure in order toreregister the patient. Interruptions in tracking may be caused by anocclusion of a trackable marker or a sudden movement of a trackedobject. In the surgical system 200 shown in FIG. 4, the array 54 of thejoint positioner 100 can be utilized to continuously scan a portion ofthe patient's anatomy. Using information obtained by the array 54, theprocessing circuit can continuously register the portion of thepatient's anatomy. This continuous registration prevents the surgeonfrom having to stop a surgical procedure to reregister the patient afteran interruption of a global tracking system has caused a registrationerror.

The construction and arrangement of the systems and methods as shown inthe various exemplary embodiments are illustrative only. Although only afew embodiments have been described in detail in this disclosure, manymodifications are possible (e.g., variations in sizes, dimensions,structures, shapes and proportions of the various elements, values ofparameters, mounting arrangements, use of materials, colors,orientations, etc.). For example, some elements shown as integrallyformed may be constructed from multiple parts or elements, the positionof elements may be reversed or otherwise varied and the nature or numberof discrete elements or positions may be altered or varied. Accordingly,all such modifications are intended to be included within the scope ofthe present disclosure. The order or sequence of any process or methodsteps may be varied or re-sequenced according to alternativeembodiments. Also two or more steps may be performed concurrently orwith partial concurrence. Such variation will depend on various factors,including software and hardware systems chosen and on designer choice.All such variations are within the scope of the disclosure. Likewise,software implementations could be accomplished with standard programmingtechniques with rule based logic and other logic to accomplish thevarious connection steps, processing steps, comparison steps anddecision steps. Other substitutions, modifications, changes, andomissions may be made in the design, operating conditions andarrangement of the exemplary embodiments without departing from thescope of the present disclosure.

The present disclosure contemplates methods, systems and programproducts on any machine-readable media for accomplishing variousoperations. The embodiments of the present disclosure may be implementedusing existing computer processors, or by a special purpose computerprocessor for an appropriate system, incorporated for this or anotherpurpose, or by a hardwired system. Embodiments within the scope of thepresent disclosure include program products comprising machine-readablemedia for carrying or having machine-executable instructions or datastructures stored thereon. Such machine-readable media can be anyavailable media that can be accessed by a general purpose or specialpurpose computer or other machine with a processor. By way of example,such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROMor other optical disk storage, magnetic disk storage, other magneticstorage devices, solid state storage devices, or any other medium whichcan be used to carry or store desired program code in the form ofmachine-executable instructions or data structures and which can beaccessed by a general purpose or special purpose computer or othermachine with a processor. When information is transferred or providedover a network or another communications connection (either hardwired,wireless, or a combination of hardwired or wireless) to a machine, themachine properly views the connection as a machine-readable medium.Thus, any such connection is properly termed a machine-readable medium.Combinations of the above are also included within the scope ofmachine-readable media. Machine-executable instructions include, forexample, instructions and data which cause a general purpose computer,special purpose computer, or special purpose processing machines toperform a certain function or group of functions.

What is claimed is:
 1. A motorized joint positioner, comprising: a basefixed to a table; a first holder configured for a first portion of apatient's anatomy to rest thereon, wherein the first holder is coupledto the base and wherein the base comprises two base joints providingmovement of the first holder in two degrees of freedom relative to thebase; a second holder configured for a second portion of the patient'sanatomy to rest thereon; a local tracking system configured to track thefirst portion of the patient's anatomy and the second portion of thepatient's anatomy relative to the motorized joint positioner, whereinthe local tracking system comprises a first detection device coupled tothe first holder and a second detection device coupled to the secondholder, a first trackable element configured to be fixed to the firstportion of the patient's anatomy, and a second trackable elementconfigured to be fixed to the second portion of the patient's anatomy;and a robotic arm coupled between the base and the second holder;wherein the robotic arm comprises at least two joints providing movementof the second holder in at least two degrees of freedom, and wherein atleast one of either of the base joints and the robotic arm includes anactuator controllable to position the corresponding first or secondholder; and wherein the motorized joint positioner is configured toapply a first gravity compensating force to the first holder tocounteract a weight of the first portion and a second gravitycompensating force to the second holder to counteract a weight of thesecond portion.
 2. The motorized joint positioner of claim 1, whereinthe actuator is a series elastic actuator.
 3. The motorized jointpositioner of claim 1, wherein the robotic arm is configured to positionthe second holder in six degrees of freedom.
 4. The motorized jointpositioner of claim 1, wherein the first portion of the patient'sanatomy is an upper leg of the patient and the first holder is a cuffconfigured to hold the upper leg of the patient, and wherein the secondportion of the patient's anatomy is a lower leg of the patient and thesecond holder is a cuff configured to hold the lower leg of the patient,such that the motorized joint positioner is configured to position aknee of the patient.
 5. The motorized joint positioner of claim 1,wherein at least one of either of the base joints and the robotic arm isforce controllable.
 6. The motorized joint positioner of claim 1,wherein the motorized joint positioner is backdrivable such that a usercan manually adjust the position of the first and second holders.
 7. Themotorized joint positioner of claim 1, wherein the motorized jointpositioner is configured to be controlled by a processing circuit toadjust the position of the first and second holders.
 8. The motorizedjoint positioner of claim 1, wherein the motorized joint positioner isconfigured to apply a known torque to a patient's joint.
 9. Themotorized joint positioner of claim 1, wherein the local tracking systemfurther comprises a local tracking device mechanically coupled to themotorized joint positioner and configured to track the first, thesecond, or a third portion of a patient's anatomy relative to themotorized joint positioner.
 10. The motorized joint positioner of claim1, further comprising a two-dimensional ultrasound array mechanicallycoupled to the first holder and configured to scan the first, thesecond, or a third portion of the patient's anatomy to enable aprocessing circuit to register the first, second, or third portion ofthe patient's anatomy to a three-dimensional representation of thefirst, second, or third portion of the patient's anatomy.
 11. The systemof claim 1, wherein the motorized joint positioner is configured toapply the gravity compensating force based on the weight of the firstportion and the second portion and on sensed incremental changes in aforce applied by a user during repositioning of the motorized jointpositioner.
 12. A motorized joint positioner system, comprising: amotorized joint positioner having: a base fixed to a table, a firstholder configured for a first portion of a patient's anatomy to restthereon, wherein the first holder is coupled to the base and wherein thebase comprises two base joints providing movement of the first holder intwo degrees of freedom relative to the base; a robotic arm coupledbetween the base and a second holder configured for a second portion ofthe patient's anatomy to rest thereon, wherein at least one of either ofthe base joints and the robotic arm includes a series elastic actuator;a local tracking system configured to track the first portion of thepatient's anatomy and the second portion of the patient's anatomyrelative to the motorized joint positioner, wherein the local trackingsystem comprises a first detection device coupled to the first holderand a second detection device coupled to the second holder, firsttrackable element configured to be fixed to the first portion of thepatient's anatomy a, and a second trackable element configured to befixed to the second portion of the patient's anatomy; a processingcircuit configured to position the motorized joint positioner bycontrolling the series elastic actuator; wherein the motorized jointpositioner is backdrivable such that it can be manually repositioned bya user; and wherein the motorized joint positioner is configured toapply a first gravity compensating force to the first holder tocounteract a weight of the first portion and a second gravitycompensating force to the second holder to counteract a weight of thesecond portion.
 13. The system of claim 12, wherein the robotic arm isconfigured to position the second holder in six degrees of freedom. 14.The system of claim 12, wherein the first holder is configured for thefirst portion of the patient's anatomy to rest thereon, and wherein thefirst portion of the patient's anatomy is an upper leg of the patientand the first holder is a cuff configured to hold the upper leg of thepatient, and wherein the second holder is configured for the secondportion of the patient's anatomy to rest thereon, and wherein the secondportion of the patient's anatomy is a lower leg of the patient and thesecond holder is a cuff configured to hold the lower leg of the patient,such that the motorized joint positioner is configured to position aknee of the patient.
 15. The system of claim 12, wherein the motorizedjoint positioner is configured to apply a known torque to a joint of thepatient.
 16. The system of claim 12, wherein the motorized jointpositioner includes a two-dimensional ultrasound array configured toscan the first portion or the second portion of the patient's anatomy,and wherein the processing circuit is configured to register the firstportion or the second portion of the patient's anatomy to athree-dimensional representation of the portion or the second portion ofthe patient's anatomy.
 17. The system of claim 16, wherein theprocessing circuit is configured to continuously register the first, thesecond, or a third portion of the patient's anatomy.